Deterrence and toxicity of plant saponins for the pea aphid acyrthosiphon pisum harris.

Plant saponins are widely distributed among plants and have a wide range of biological properties. Three alfalfa saponins - zanhic acid tridesmoside, 3GlcA, 28AraRhaXyl medicagenic acid glycoside, and 3GlcA, 28AraRha medicagenic acid glycoside - were tested for their settling inhibition effects on feeding behavior of the aphid Acyrthosiphon pisum using the electrical penetration graph method. Application of saponins to artificial diets affected the insects' probing behavior. In general, saponins incorporated into sucrose-agarose gels significantly reduced the number of aphid probes and extended their duration. Lower saponin concentrations (50 ppm) extended aphid activity and corresponded to phloem sap ingestion. In contrast, higher concentrations (100 ppm) strongly reduced aphid ability to ingest phloem and xylem sap.